The importance of secondary cooling, as concerns not only the quality of the cast products but also the productivity of the casting machine, has long been recognized. The results to be achieved by a good regulation of the cooling rate include:
complete solidification of the product before a certain operating stage, especially at a point where a slab descending along an arcuate path is straightened before being subjected to blow-torch cutting;
good mechanical strength of the solidified skin of the product along the machine and, in particular, avoidance of swelling problems due to an excessive surface temperature which may cause significant cracks and central segregation;
a substantially even cooling of the product so as to eliminate any sudden temperature change likely to create so-called median cracks in the solidification front; and
maintenance of the surface temperature during straightening in the range of good forgeability of the metal to avoid the formation of transverse cracks on the concave side of a curved slab.
Under steady operating conditions, the optimum setting of the rate of secondary cooling involves, on the one hand, a suitable distribution of the spray water along the cast product or workpiece in the several cooling zones of the machine and, on the other hand, an optimization of another parameter in direct relationship with the productivity of the machine, namely the casting speed. This optimization is satisfactorily accomplished with the present state of the art.
Under varying working conditions necessitating changes in the casting speed, however, optimization of cooling requires a controlled variation of the water-flow rates satisfying at all times, as much as possible, the above-mentioned desiderata.
Different modes of controlling the secondary cooling have already been proposed. They all use the casting speed as an active parameter for calculating the water-flow rates, but may be separated into different groups depending on the method adopted. More particularly, in a first group of methods the flow rates are determined in the different spraying zones solely as a function of the instantaneous casting speed. These methods are, generally, poorly adapted to the casting of slabs for the production of thick sheets; thus, for example, they generally do not allow, upon a sudden slowdown of the casting, the surface temperature of the slab to be maintained in a range best suited for forging products of the chosen grade.
In a second group of methods, these flow rates are determined on the basis of an average speed derived from the past and present history of the casting operation.
These methods are then based on:
(a) The definition of a parameter characterizing in each spraying zone the past and present history of the cast product. In most cases, with an elongate workpiece such as a slab conceptually divided into a succession of unit-length elements, it is a question of the average age of the element present at any time in a given zone; the age of a unit-length element is defined as the time (residence time) spent by it in the machine from its creation in the ingot mold.
(b) The choice of a spraying curve based on metallurgical criteria and indicating for each zone the flow rate of the spray water in dependence upon the value of the aforementioned parameter.
The latter method, accordingly, involves a varying water distribution among the different zones and generally requires the use of a computer because of the numerous calculations to be made so as to determine, at regular time intervals, the average age of the elements in these zones.
The methods of this group differ from one another by the choice of the spraying curves and the cooling criteria to which they conform, as well as by the mode of calculating the average age.
Reference may be made in particular to French patent application No. 80/05592 and corresponding European application 36.342, published Sept. 23, 1981, as well as to a French-language article by J. Foussal, published June 1978 in Revue de Metallurgie, pages 403-414. The system described in the French application includes a computer storing data for two families of curves, pertaining to different casting speeds, which respectively represent heat extraction and surface temperature in different irrigation zones and serve as reference parameters for the adjustment of the spray. The Foussal article discusses the calculation of the mean or average age of a workpiece element to be coated.
In all instances, the spraying curves are to be chosen so as to best attain the objectives of cooling, in particular of maintaining the surface temperature in a straightening zone above the poor-forgeability range of the cast product; in steel casting, generally, this temperature should be not less than about 900.degree. C. for avoiding the formation of transverse cracks on the inner or concave side of a slab.
Even the most perfect conventional systems for controlling the secondary cooling still do not reliably attain this objective, mainly because of significant transitory conditions inherent in continuous casting, such as change of steel grade, replacement of pouring vessels, as well as start-up and end of casting.
This is all the more true since the last cooling zones have only limited ranges of adjustability. In general, moreover, the last zone upstream of a straightening stage is often deprived of cooling means. Also, should substantial speed variations occur, little can be done to correct the thermal profile of the part of the workpiece situated in these terminal zones. This may not be very serious if the change consists in an acceleration since in that case the temperature increases; its rise, however, should not be carried too far, because of the risks of swelling or rupture of a workpiece which is not yet completely solidified.
On the other hand, the situation may become critical should the workpiece slow down or stop as, in this instance, the temperature drops unavoidably and may fall into the poor-forgeability region, even if cooling is halted, simply through heat loss by radiation.